Steel tube surface treatment

The surface treatment of steel pipes plays a crucial role in ensuring the effectiveness and durability of the epoxy powder coating. To achieve optimal results, the anti-corrosion production line utilizes two shot blasting and descaling machines connected in series. The first machine employs steel shots to effectively remove rust and scale from the pipe surface. The second machine uses steel grit to create a roughened texture, which enhances the adhesion of the powder coating. Both the steel shots and grit are sourced from France, known for their high-quality materials. The hardness of the steel shots is between 48-52 HRC, while the steel grit has a slightly higher hardness of 53-57 HRC. The selection of these abrasives considers factors such as the strength and hardness of the steel pipe, as well as the reusability of the media. The number of times they can be reused and their impact on the lifespan of the blast nozzles are also taken into account. After processing through both machines, the surface cleanliness of the steel pipe reaches a standard of Sa2.5 to Sa3.0, with an anchor profile depth of approximately 80 micrometers—well within the required range of 40-100 micrometers. Phosphoric acid is used for pickling, with a concentration set at 10% (the manufacturer recommends 5-20%). For chromate treatment, the concentration is also maintained at 10%, though the factory suggests a broader range of 5-50%. The salt content on the steel pipe surface is tested using chemical reagents to ensure proper cleaning. The heating temperature during the coating process depends on the pipe's diameter, wall thickness, and the thickness of the anti-corrosion layer. Based on the manufacturer’s recommendations, the temperature is set between 235°C and 245°C. The production speed is adjusted according to the gelation and curing time of the powder. A differential scanning calorimeter (DSC) is used to measure the curing time, which is provided by the manufacturer: 605 seconds at 235°C and 405 seconds at 245°C. This ensures that the coating meets the necessary curing requirements. A total of 24 spray guns are used during the application process. The flow control system is designed to maintain even coating thickness, ensuring that the majority of the powder adheres to the pipe surface. In case of defects, a two-component liquid powder repair material is applied. The repair process involves sanding or filing the damaged area and its surroundings to create a smooth surface. The mixed repair material is then applied using a spatula, ensuring it is evenly spread and slightly thicker than the surrounding coating. Once the surface is dry, an EDM inspection is conducted to check for any leakage points. If no leaks are detected, the repaired section is considered合格 (qualified).

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